1. Field of the Invention
This invention relates to a tire using a rubber-steel cord composite, and more particularly to an improvement of a plated portion of a steel cord between a plated steel cord and a coating rubber.
2. Description of Related Art
A rubber composition applied as a coating rubber to a belt layer of a tire reinforced with, for example, steel cords is an important rubber contributing to a durability of the tire. As a property required in such a rubber composition, there are (1) high hardness, (2) low heat build-up, (3) good adhesion to steel cord, (4) high resistance to deterioration and so on. Among these performances, the increase of the hardness (1) is an effective means for suppressing shearing strain between belt layers to improve the durability of the tire.
As a method of increasing the hardness, there have hitherto been made (a) a method of increasing a compounding amount of a filler such as carbon black or the like, (b) a method of adding a resin or the like, (c) a method of increasing a compounding amount of a crosslinking agent such as sulfur or the like, (d) a method of increasing a compounding amount of a vulcanization accelerator, and so on.
In the methods (a) and (b), however, the hardness is certainly increased, but the low heat build-up property lowers to degrade the thermal durability.
In the method (c), when the rubber composition is left at an uncured state, sulfur blooms out to considerably lower the workability, and also even if a tire could be manufactured, the resistance to deterioration of rubber lowers and hence the tire durability may be degraded.
In the method (d), when the amount of the vulcanization accelerator compounded is increased up to a certain level, there are not cause problems in the workability, low heat build-up property and resistance to deterioration, but if the amount is too large, there may be caused a problem that the adhesion to the steel cord lowers.
It is, therefore, an object of the invention to provide a tire having an excellent durability by improving a rubber-steel cord composite.
The inventors have made various studies in order to achieve the above object and found the following knowledge and, as a result, the invention has been accomplished.
That is, it is considered that bismaleimide directly crosslinks polymers such as natural rubber or the like therebetween without using a sulfur, so that the hardness of a rubber composition can be increased without damaging the low heat build-up property and the resistance to deterioration in rubber. And also, it has been found that such an effect can be obtained at a certain level by the addition of the bismaleimide alone, but when the bismaleimide is added together with a trans-polybutadiene, a reversion liable to be increased by the bismaleimide can be suppressed to further improve the high hardness and low heat build-up property. Furthermore, it has been found that the adhesion between a rubber composition and a steel cord can be improved by adding a bismaleimide alone or together with a trans-polybutadiene.
On the other hand, a cobalt salt, which serves as an adhesion promoter in the adhesion between a rubber composition and a steel cord, is required in order to promote the adhesion. But it also possesses an action of degrading the resistance to deterioration of a rubber composition. In the invention, it is possible to decrease an amount of a cobalt salt by using a bismaleimide alone or by using a bismaleimide and a trans-polybutadiene when a cobalt salt is used in the rubber composition.
Further, it has been found that the adhesion can be more improved by combining the above rubber composition with a novel steel cord as mentioned later.
According to the invention, there is the provision of a tire using a rubber-steel cord composite of a steel cord and a rubber composition comprising at least one rubber ingredient selected from natural rubber and synthetic diene rubbers, and a bismaleimide compound, an amount of the bismaleimide compound being compounded of 0.1-5 parts by weight based on 100 parts by weight of the rubber ingredient.
In the invention, it is favorable that a trans-polybutadiene is further compounded in an amount of 0.1-15 parts by weight based on 100 parts by weight of the rubber ingredient, that the rubber ingredient contains not less than 50% by weight of natural rubber, and that the bismaleimide compound is represented by the following general formula (I): 
wherein R is an aromatic group having a carbon number of 6-18 or an alkyl-aromatic group having a carbon number of 7-24, and x and y are an integer of 0 to 3, respectively.
And also, it is favorable that the steel cord is a brass-plated mono-filament steel cord comprising one steel filament containing at least one of cobalt atom and nickel atom in a surface layer region ranging from a surface of a steel filament plated with a brass up to a depth of 15 nm inward in a radial direction of the filament and having a surface copper concentration of 15-45 atomic %, or a multifilament steel cord obtained by twisting a plurality of the above steel filaments.
Further, it is favorable that a total amount of cobalt atom and nickel atom contained in the surface layer region is not less than 0.1 atomic % but not more than a content of copper atom, preferably 0.5-5.0 atomic %, and that cobalt atom and nickel atom not contained as an oxide in the surface layer region is not less than 50 atomic % of the total amount of cobalt atom and nickel atom contained in the surface layer region, and that an average thickness of the plated brass is 0.13-0.30 xcexcm, and that a diameter of the steel filament is not more than 0.40 mm.